Breathable underlay, also sometimes called breathable roofing felt, comes in two types:
- Vapour permeable underlay
- Vapour and air permeable underlay
The two types are often mistakenly grouped together as one, without being properly distinguished for their differing capabilities.
Vapour permeable underlays
Vapour permeable underlays are usually the cheaper of the two types. The fibrous structure of vapour permeable underlays is sufficiently dense to prevent liquid water from penetrating, while allowing water vapour to diffuse.
Although water vapour can diffuse, it is often still necessary to have additional ventilation to carry this vapour out of the roof space. The NHBC has recently implemented guidelines that state when using a vapour permeable underlay, there should also be high-level ventilation to provide sufficient airflow to draw this vapour out of the building.
Vapour and air permeable underlays
Vapour and air permeable underlays, also known as air open underlays, are generally a more expensive form of roof underlay.
Air open underlays have the lowest vapour resistance and negate the requirement for any additional roof ventilation (according to their manufacturers). Whilst the claim is supported by these manufacturers, there are still some questions over the long-term performance of these types of underlays and their suitability when specified with a close-fitting roof covering. In all instances, anyone wishing to install this underlay type should follow the recommendations of the roof tile manufacturer.
Other ventilation considerations
Where an external covering (such as fibre cement slates) is relatively airtight, there is a risk of interstitial condensation forming on the underside of the underlay and the external covering.
To avoid that risk, the batten space should be vented (see BS 5250).
Due to the increased airflow properties of this underlay, there is also some question over its long term impact on the thermal performance of the roof space.
In all cases, the specification or recommendations of the roof tile manufacturer should be followed.